Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/54—Contact plating, i.e. electroless electrochemical plating
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/1601—Process or apparatus
  • C23C18/1617—Purification and regeneration of coating baths
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/1601—Process or apparatus
  • C23C18/1633—Process of electroless plating
  • C23C18/1646—Characteristics of the product obtained
  • C23C18/165—Multilayered product
  • C23C18/1653—Two or more layers with at least one layer obtained by electroless plating and one layer obtained by electroplating
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/22—Secondary treatment of printed circuits
  • H05K3/24—Reinforcing the conductive pattern
  • H05K3/244—Finish plating of conductors, especially of copper conductors, e.g. for pads or lands

Definitions

• This invention relates to the chemical displacement plating. More particularly, this invention relates to plating of tin on copper, copper alloys, and other metals by chemical displacement using a spray or cascade application process. Still more particularly, this invention relates to the use of such chemical displacement plating in the manufacture of printed circuit boards.
• Coatings of tin typically have been applied to surfaces of copper and copper based alloys by a particular mode of displacement plating, i.e., immersion plating techniques such as disclosed in U.S. Pat. No. 2,891,871, U.S. Pat. No. 3,303,029 and U.S. Pat. No. 4,715,894.
• Dislacement plating has also been referred to as "replacement” plating and for the purpose of this invention the terms are intended to be synonymous.
• a bath is prepared containing an aqueous solution of a tin(II) salt, an acid, and thiourea or a thiourea derivative as essential ingredients.
• an article bearing a copper surface e.g., a copper clad printed circuit board
• the plating bath for a period of time during which the surface copper metal is oxidized to copper(I) ion and complexed with the thiourea and is replaced at the surface by the concurrently reduced tin metal from the tin(II) ion.
• the article is removed from the bath and is rinsed to remove residual plating solution.
• concentration of copper(I) thiourea complex in the immersion bath increases.
• PCB's printed circuit boards
• Printed circuit boards comprise a non-conducting or dielectric such as a fiberglass/epoxy sheet which is clad with a metal conductive layer such as copper on either one or both surfaces.
• the metal layer on the PCB before processing typically is a continuous layer of copper which may be interrupted by a pattern of plated through holes or vias linking both surfaces of the board. During processing selected portions of the copper layer are removed to form a raised copper circuit image pattern of the PCB.
• Multilayer PCB's are typically constructed by interleaving imaged conductive layers such as one containing copper with dielectric adhesive layers such as a partially cured B-stage resin, i.e., a prepreg, into a multilayer sandwich which is then bonded together by applying heat and pressure. Production of these types of printed circuit boards are described in "Printed Circuits Handbook", Third Edition, edited by C. F. Coombs, Jr., McGraw-Hill, 1988, which is incorporated herein by reference. Since a conductive layer with a smooth copper surface does not bond well to the prepreg, several copper surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich.
• One such copper surface treatment is the use of immersion tin and tin alloys as a bonding medium for multilayer circuits as disclosed by Holtzman et al., U.S. Pat. No. 4,715,894.
• an immersion tin composition is disclosed containing both thiourea compounds and urea compounds to displacement plate the copper surface of each PCB with tin by the immersion process prior to laminating them to form a multilayer board.
• bond strength of multilayer PCB's prepared by this immersion process was improved, the production efficiency of multilayer PCB's is limited by the batch process wherein substantial quantities of plating bath is lost through drag-out of the solution with each PCB processed.
• the PCB's made by this immersion process are susceptible to defects due to streaking described supra.
• the in-line process disclosed includes a spray displacement tin plating step followed by a post-treatment step with a silane bonding mixture of a ureido silane and a disilyl crosslinking agent.
• PCB's are fed by conveyor through a series of treatment and rinse stations in which the PCB's are sequentially cleaned, microetched, spray tin displacement plated, post-treated with the silane bonding mixture and dried.
• the PCB's prepared by this spray tin displacement plating system are substantially free of streak defects observed in the immersion batch process and the multilayer PCB's prepared therefrom demonstrate improved resistance to delamination during typical high temperature soldering operations.
• the plating solution is sprayed onto the PCB and the excess solution is recovered and returned to the plating bath sump with minimal drag-out to succeeding rinse stations.
• improved multilayer PCB's have been obtained by the disclosed process, it has now been observed that the activity of the plating bath solution declines during use due to the accumulation of tin(IV) ion formed by aerial oxidation of tin(II) during the spray application step.
• the concentration of copper(I) thiourea complex increases in the recirculated plating solution until its solubility limit is surpassed and crystalline complex is precipitated which clogs the spray nozzles and interferes with the mechanical components of the plating system.
• the life of the displacement plating bath has been extended and formation of complex precipitate therein prevented by the present invention which is a process for displacement plating a substrate metal surface with an other metal comprising the steps:
• step (d) repeating steps (b) and (c) for a series of substrate metal surfaces whereby the concentration of the substrate metal ion complex in the plating solution reaches a high level which is below the concentration at which the ion complex precipitate is formed; and when the concentration of the substrate metal ion complex in the plating solution reaches the high level,
• step (j) repeating steps (e) through (h) a sufficient number of times until the concentration of the substrate metal ion complex in the plating solution drops to a predetermined low level.
• the activity of the plating solution may be maintained by adding sufficient complexing agent to compensate for its removal as a substrate metal ion complex.
• the reservoir of aqueous plating solution contains free metal (iv) which is the free metal of the metal ions present in their lowest oxidation state (i), whereby at least a portion of the metal ions present in their higher oxidation state are reacted with free metal (iv) to form metal ions present in their lowest oxidation state to replenish the aqueous plating solution.
• the substrate metal or its alloy is reclaimed from its ion complex and the freed complexing agent is recycled into the displacement plating solution by the added process wherein the following steps are added:
• step (m) replenishing the aqueous plating solution of step (a) with the aqueous acid solution containing the reformed complexing agent.
• the present invention is directed to the stabilization and replenishment of displacement plating processes in which the plating solution is sprayed, cascaded, poured onto or otherwise applied in the presence of air to the substrate surface to be plated.
• the present invention is also directed to the reduction of waste generated by such a plating process.
• Displacement plating solutions include immersion tin and tin alloy solutions such as disclosed in Holtzman et al., U.S. Pat. No. 4,715,894, which is incorporated herein by reference.
• the displacement metal plating process does not employ an electric current but is based on an electrochemical displacement reaction.
• the metal substrate that is to be plated generally is more active (less noble) than the metal salt that is dissolved in the coating composition or plating solution. Copper may be plated by tin solution even though copper is more noble than tin when the immersion coating composition is acidic and contains thiourea as a so-called complexing agent.
• Metal ions contemplated for use in the present invention generally are simple cations of the metal salt, e.g., tin(II) and tin(IV) ions.
• Displacement tin plating solutions are particularly susceptible to aerial oxidation. Consequently, application of such solutions typically has been limited to immersing or dipping substrates into the plating solution, thereby minimizing aerial oxidation of the plating bath.
• a spray displacement tin plating process for bonding multilayer printed circuit boards has been disclosed in Palladino, U.S. Pat. No. 5,073,456 (assignee's U.S. patent application Ser. No. 07/446335, filed Dec. 5, 1989), and in a publication in "Printed Circuit Fabrication", Vol. 13, No. 5, Pages 46-60 May 1990, by K. H. Dietz, J. V. Palladino and A. C.
• the present invention will be described in the context of a spray displacement tin plating process particularly for the manufacture of multilayer printed circuit boards but it is not intended to be limited thereby.
• the multilayer printed circuit board has alternating layers of dielectric material which support copper circuitry (which may have interspaced other layers such as a copper sheet which serves as a ground plane) which are adhered to an insulating layer through intermediate layers.
• the circuit board has conductive through holes which form electrical paths across the entire thickness of the board.
• multilayer circuit boards several dozen conductive and nonconductive layers can be employed. Also, for formation of multilayer circuit boards, it is necessary to drill holes and defects can occur due to delamination of layers in the areas immediately surrounding a hole. If a defect is present in one of the layers or if delamination occurs, generally the entire board must be scrapped. Therefore, high quality in each of the steps of formation of the printed circuit board is essential for commercial production.
• One such step for forming high quality multilayer boards is the formation of defect free tin plating over the copper circuitry of each constituent board.
• a starting material in the present invention is a dielectric layer which contains on one or opposite surfaces a cladding of copper.
• This copper layer is of a thickness of at least 4 microns and more preferably 32 microns and it is used to form conductive circuitry. Well known techniques can be employed to form such circuitry such as described in Coombs supra.
• the composition of the dielectric layer is not critical provided it functions as an electrical insulator.
• a partially cured thermosetting polymer composition is employed which is known in the art as prepreg or "B" stage resin.
• the circuitry of the printed circuit board typically is first cleaned and etched, such as disclosed in Palladino supra.
• the cleaned and etched printed circuit board is then tin plated using a process such as that disclosed in Assignee's copending U.S. Patent Application, U.S. Ser. No. 07/799,135, which is a process for displacement plating a copper surface of a printed circuit board with tin or a tin alloy comprising the steps: (a) providing a reservoir of aqueous displacement tin plating solution comprising;
• the ratio of the surface area of the free tin metal to the volume of the aqueous displacement tin plating solution is at least 4 in 2 /gallon (6.8 cm 2 /liter); (b) spraying a stream of the aqueous displacement tin plating solution from the reservoir onto the copper surface; whereby a portion of the tin(II) ion is aerially oxidized to a tin(IV) ion, and whereby the tin(II) ion is reduced to the free metal to displace surface copper which is oxidized to copper(I) ion and complexed with the thiourea to form a copper(I) thiourea complex dissolved in the reacted aqueous displacement tin plating solution at the surface of the copper; and (c) returning the sprayed and reacted aqueous displacement tin plating solution to the reservoir so that the portion of the tin(IV) ion formed is reacted with the surface of
• thiourea may be added to replenish the plating solution.
• the precipitated copper(I) thiourea complex produced by this process may be disposed of using conventional waste-treatment processes such as that disclosed by Dietz et al. supra. Since thiourea is an objectionable waste, a waste-treatment process using hydrogen peroxide was developed for its treatment to reduce concentrations to less than 1 ppm.
• the copper or its tin alloy may be reclaimed from precipitated copper(I) thiourea complex and the freed thiourea may be recycled to replenish the aqueous displacement tin plating solution by the added process of this invention wherein the following steps are added: (k) dissolving the removed copper(I) thiourea complex precipitate in an aqueous acid solution to form a redissolved copper(I) thiourea complex solution wherein the acid is the same acid (iii) as in the aqueous displacement tin plating solution; (1) electroplating the copper onto a cathode from the redissolved copper(I) thiourea complex solution contained in an electrolytic cell having the cathode and an anode, to reform the thiourea in the aqueous acid solution, wherein the electrolytic cell is configured so that the reformed thiourea is isolated from the anode, e.g., by a salt bridge, semipermeable membrane, a frit
• the spray displacement tin plating process described by these added steps is virtually a closed loop plating system with a very long plating solution life.
• the displacement tin plating solution is substantially replenished by the quantity of free tin metal added, i.e., the tin surface area in the solution, and the strength of the acid solution used to dissolve the complex precipitate.
• the only substantial by-product would be the electrowinned copper alloy (bronze) with little or no waste treatment needed for rinse water.
• step (f) An alternate process to the disposal of the copper(I) thiourea complex precipitate formed in step (f) above is an electrowinning process of this invention wherein the following steps are added after step (g): (k) dissolving the removed copper(I) thiourea complex precipitate in an aqueous acid solution to form a redissolved copper(I) thiourea complex solution; (n) electroplating the copper onto a cathode of an electrolytic cell containing the cathode and an anode, from the redissolved copper(I) thiourea complex solution contained therein to reform the thiourea so that the reformed thiourea is oxidized at the anode to form an acid solution of the oxidized components of the thiourea; and (o) disposing of the acid solution of the oxidized components of the thiourea.
• the displacement tin plating solution is partially replenished by the quantity of free tin metal added, i.e., the tin surface area in the solution and additional acid thiourea solution would be needed to complete the replenishment.
• the copper or copper alloy (bronze) is reclaimed from the copper(I) thiourea complex and the resulting thiourea is oxidized for disposal as an acid waste by conventional waste treatment procedures.
• the aqueous displacement tin plating solution may contain additional components such as urea, reducing agents, surfactants and the like as disclosed in Holtzman et al. supra and Palladino supra.
• additional components such as urea, reducing agents, surfactants and the like as disclosed in Holtzman et al. supra and Palladino supra.
• a salt of a second metal such as lead, is present in the solution.
• the aqueous displacement tin plating solution contains a thiourea compound, a tin(II) salt, a reducing agent, an acid and a urea compound.
• the tin(II) salts of an inorganic (mineral) acid such as the sulfur, phosphorous and halogen acids may be used or an organic acid may be used (e.g., tin(II) formate, tin(II) acetate and the like).
• Preferred are the tin(II) salts of the sulfur acids such as sulfuric acid and sulfamic acid.
• Alkali metal stannates may also be used such as sodium or potassium stannate and the known equivalents thereof.
• lead acetate may be used as the lead salt.
• thiourea compounds that are used may be either thiourea or the various art known derivatives, homologes, or analogs thereof such as disclosed in columns 11 and 12 of Holtzman et al., U.S. Pat. No. 4,715,894 which has been incorporated herein by reference supra. Thiourea is preferred.
• Free tin metal may be present in the aqueous displacement tin plating solution in any form, e.g., extruded tin, "mossy" tin, cast tin, and the like. Extruded tin, such as the tin slabs conventionally used as electrolytic anodes or tin wire, is preferred since the amount needed to control stabilization of the solution is easily adjusted by removing or adding portions of the tin to achieve the desired surface to volume ratio.
• the ratio of the tin surface to the volume of the aqueous displacement tin plating solution typically will be at least about 4 2 /gallon (6.8 cm 2 /liter) and preferably about 16 2 /gallon (27.2 cm 2 /liter) or greater.
• the acids that are used may be organic acids or inorganic acids (mineral acids) based on sulfur, phosphorous, the halogens, or the mixtures thereof, the sulfur based mineral acids being preferred such as sulfuric acid and sulfamic acid. Particularly preferred is the mixture of sulfuric acid and hypophosphorous acid.
• Some of the organic acids that may be used comprise monocarboxylic or dicarboxylic acids having up to about six carbon atoms such as formic acid, acetic acid, malic acid, maleic acid, and the like.
• halogen acids or halogen salts it is preferred, if possible, not to use halogen acids or halogen salts since halide residues will be produced in the tin coating deposited.
• Halide salts interfere with electrical properties of the tin and may also act as corrosive materials in the coating.
• the urea compound that may be used may be either urea or the various art known derivatives, homologes, or analogs thereof such as disclosed in columns 12 through 15 of Holtzman et al., U.S. Pat. No. 4,715,894 which has been incorporated herein by reference supra. Urea is preferred.
• Chelating agents that may be used generally comprise the various classes of chelating agents and specific compounds disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 5, pages 339-368 incorporated herein by reference. Chelating agents that are especially preferred comprise aminocarboxylic acids and hydroxycarboxylic acids. Some aminocarboxylic acids that may be used comprise ethylenediaminetetraacetic acid, hydroxyethyl-ethylenediaminetriacetic acid, nitrilotriacetic acid, N-dihydroxyethylglycine, and ethylenebis(hydroxy-phenylglycine). Hydroxy carboxylic acids that may be used comprise tartaric acid, citric acid, gluconic acid and 5-sulfosalicylic acid.
• the various reducing agents that may be used are well known in the art and generally comprise organic aldehyde whether saturated or unsaturated, aliphatic or cyclic, having up to about ten carbon atoms.
• Lower alkyl aldehydes having up to about six carbon atoms may be employed in this respect such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and the like.
• Especially preferred aldehydes comprise hydroxy aliphatic aldehydes such as glyceraldehyde, erythrose, threose, arabinose and the various position isomers thereof, and glucose and the various position isomers thereof.
• Glucose has been found to act to prevent oxidation of the metal salts to a higher oxidation state, e.g., tin(II) ion to tin(IV) ion, but also as a chelating agent and is especially useful for these reasons.
• the surfactants that may be used comprise any nonionic, anionic, cationic or amphoteric surfactant such as those listed in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 22, pages 332-387 incorporated herein by reference.
• the nonionic surfactants are especially preferred.
• the various components of the aqueous displacement plating solution may be present at conventionally established concentrations.
• the displacement plating solution will contain on a molar basis:
• the solution may also contain on a molar basis;
• the solution concentrations may, of course, vary depending on the particular plating application intended.
• the tin coated copper circuitry of each component circuit board is further treated to form a thin layer of an oxide, hydroxide or combination thereof on the surface of the tin in order to improve the bonding to the interleaved dielectric layers.
• the treated tin surface is further treated with a silane bonding mixture to further improve bonding of the component layers of the multilayer circuit board during throughout its manufacture and end use life.
• the silane bonding mixture is a mixture of a ureido silane and a disilyl crosslinking agent which was disclosed in Palladino supra.
• the stabilized spray displacement plating process of this invention may be used in other plating applications, e.g., as an etch resist in the manufacture of printed circuit boards.
• a polymeric or resin resist image is first formed on a copper clad circuit board substrate and then a metal resistant to etchants is plated on the copper surface areas not protected by the polymer resist image to form a complimentary metal resist image.
• the polymer resist image is then stripped from the copper surface and the uncovered copper not protected by the metal resist image is removed from the substrate by an etchant to form the printed circuit.
• immersion tin coatings as an etch resist in the plate-and-etch process is disclosed in Holtzman et al., U.S. Pat. No. 4,657,632 which is incorporated herein by reference wherein an etch resist immersion tin composition is selectively applied to the metal layer to leave areas of coated and uncoated metal followed by etching the metal not coated with the resist.
• immersion tin composition is applied as a substantially pore free coating at thicknesses from about 0.08 to about 0.175 microns.
• Holtzman et al. '632 further discloses that such immersion tin coatings overcome deficiencies in conventional electroplated tin-lead resists during subsequent soldering operations.
• a solder mask is first applied to the printed circuit board to cover all board areas except where components are to be soldered thereto.
• the electrolytically deposited tin-lead etch resist on the circuit is removed by reflowing it at elevated temperatures and since the removal is not always uniform the circuit board sometimes has to be subjected to a leveling process.
• a leveling process comprises passing the board over a hot air knife, i.e., a constricted elongated hot air jet.
• Holtzman et al. '632 disclose that when immersion tin coatings are used as the resist, the reflow and hot air leveling steps can be eliminated.
• the stabilized spray displacement plating process of this invention and its equivalents described supra may be used to produce superior etching resists for the plate-and-etch manufacture of circuit boards.
• the aqueous displacement plating solution of this invention will contain a water soluble salt of the displacement metal ion present in its lowest oxidation state.
• Such metal salts comprise those based on metals of group IVA; VB; VIB; VIIB; IB; IIB and IIIA of the Periodic Table of the Elements; the group IVA, VIII, IB, IIB, and IIIA metals being preferred; and the group IVA, VIII, and IB metals being especially preferred.
• Preferred metals which fall into this class are tin, lead, mercury, nickel, gold, silver, indium, germanium and palladium.
• the anions of these metal salts are the same as those defined herein for the tin salts.
• Particularly preferred are tin and various combinations of tin and other metals such as tin-lead, tin-nickel, tin-mercury and the like.
• the metal salts as defined above and herein are typically employed in their lowest oxidation states, e.g., stannous tin(II); nickelous Ni(II); mercurous Hg(I); aurous Au(I) and the like.
• tin in its lowest oxidation state whereas any of the other metal salts may be employed in any oxidation state.
• Various mixtures of these other metal salts may also be employed. Salts of germanium, lead, mercury, silver, indium, gold and palladium are especially suitable.
• displacement plating of the copper printed circuit may be deferred until after the solder mask has been applied, or displacement plating may be repeated prior to the soldering operation. Such deferral or repetition can improve the solder wetability of the plated connection sites during assembly and soldering of components to the circuit.
• Innerlayers for the manufacture of multilayer ⁇ , treated printed circuit boards were chemically cleaned with a displacement tin composition and a silane bonding mixture in an in-line, conveyorized, spray treatment system such as disclosed in Palladino supra and in Dietz et al. supra.
• the in-line spray system used to prepare the innerlayer panel surfaces had a conveyor speed of 4 feet per minute and contained the following process steps and conditions.
• the alkaline cleaner used in the system was VersaCLEANR 415 (Du Pont) and the microetch was SureETCHR 550 (Du Pont) potassium peroxy monosulfate/sulfuric acid.
• Step 6 the displacement tin composition was formed by mixing Solution A and Solution B of the following compositions:
• Step 9 the silane treatment solution was prepared by adding 60 ml of glacial acetic acid to 151 liters (40 gallons) of D.I. (deionized) water. 0.83% by solution weight (1571 grams) of gamma-ureidopropyltriethoxysilane coupling agent in methanol (50%) (A-1160 Union Carbide) and 0.17% by solution weight (322 grams) of 1,2-bis(trimethoxysilyl)ethane was then added followed by sufficient deionized water to produce 189 liters (50 gallons) of solution. The solution was then mixed by activating the recirculating system of the silane treatment spray module. The solution was allowed to mix for 15 to 20 minutes to insure complete hydrolysis of the organosilane to an organosilane-triol.
• the freshly prepared displacement tin solution has a tin(II) ion concentration of about 11 g/L but during use in the spray plating process the tin(II) ion concentration and plating activity drops due to its removal as plated tin and to aerial oxidation to tin(IV) ions.
• Normal replenishment procedures could be employed to raise the tin(II) ion concentration but are ineffective in maintaining the plating efficiency at the high activity level needed for a commercial process.
• the activity level should be sufficiently high so that plated boards are substantially defect free. Since there is minimum "drag-over", i.e., removal of plating solution along with the board during transition into the rinse cycle of the system, aerial oxidation products accumulate in the reservoir.
• the displacement tin solution typically is discarded when the tin(II) ion can no longer be maintained above 2.0 g/L.
• the rate of aerial oxidation of tin(II) ion to tin(IV) ion is controlled by the amount of agitation created during the spray process and thereby is equipment dependent. Using existing equipment, the rate of tin(II) ion oxidation varies from 0.2 to 1.0 g/L per hour during spray agitation.
• a freshly prepared displacement plating solution was used until the tin(II) ion concentration reached a level of about 5 g/L.
• 12 extruded tin slabs (of the type conventionally used as electrolytic anodes) having a total surface area of 12 square feet were placed in the reservoir of the displacement tin solution and the concentration of tin(II) ion was monitored over a period of two months of normal plating use.
• the concentration of tin(II) ion stabilized at about 8.0 g/L and during the two month period, the surfaces of the tin slabs were etched away.
• the innerlayers prepared during this two month period passed AOI inspection immediately before they were layed-up in the manufacture of multilayer boards. During this time the multilayer boards produced continued to pass production qualification criteria, including thermal stress, humidity, pink ring, and adhesion testing criteria.
• Example 2 When the spray displacement plating process described in Example 1 was used over a prolonged period of time, crystals formed in the plating bath reservoir which clogged the spray nozzles and interfered with the mechanical components of the conveyor system. At this juncture the plating bath was discarded and the system cleaned and charged with fresh plating solution. The crystals were isolated and determined to be a copper(I) thiourea complex.
• the crystals formed were removed from the solution by pumping the solution through an in-line cartridge filter system which removes particles above 5 microns. The supernatant solution was fed back into the plating bath reservoir.
• Steps 1 through 3 were repeated 4 times using the same cooling/filtering system; (At this juncture of the process the concentration of the copper(I) thiourea complex in the plating bath had dropped to about 80% or lower.)
• the 10% acid solution resulting from Step 5 was disposed of by a conventional waste treatment process for thiourea, copper and acid wastes. By the removal of excess copper(I) thiourea complex using this process, the useful life of the displacement tin plating solution was more than doubled while all multilayer boards which were prepared with the plating solution passed production qualification criteria.
• one chamber was placed 30 cm of 1 mm Pt wire (working electrode) and a Ag/AgCl reference electrode, and in the second chamber, 30 cm of 1 mm Pt wire (counter electrode).
• the reference electrode provides a stable reference potential to insure the applied voltage does not drift.
• This method would allow the recovery (from the working electrode chamber) and reintroduction of the dilute acid/thiourea solution back into the displacement tin plating reservoir, while recovering copper in a recyclable form.
• the dilution caused by the extra 5 gallons of dilute acid in which the thiourea remains dissolved, would compensate for normal evaporative water loss from the plating solution.
• This example discloses an alternate process for disposing of the thiourea and copper wastes from the 10% acid solution resulting from Step 5 of Example 2.
• the reference electrode provides a stable reference potential to insure the applied voltage does not drift.
• This process allows the removal of copper in a recyclable form along with complete oxidation of thiourea to provide a solution which can be disposed of by conventional acid waste practices.
• This example demonstrates the effects of varying the thiourea and copper thiourea complex levels upon the efficacy and quality of tin plating using an immersion tin displacement plating bath.
• a solution termed “a” was prepared consisting of DI water (100 ml), concentrated sulfuric acid (50 ml), 50% hypophosphorous acid (20 ml), tin (IV) sulfate (16 g.), and additional DI water (dilution to 250 ml).
• This example indicates feasibility of crystallization for removal of copper-thiourea complex--i.e., solubility at a higher temperature (38° C.) and insolubility at a lower temperature (ambient, about 24° C.).
• An immersion displacement tin plating bath was prepared using 125 ml of "A" solution and 125 ml of "B” solution as defined in Example 1. The bath was used to plate 200 2 ⁇ 2" coupons of copper laminate individually with 90 second immersion time at ambient temperature. DI water was added periodically to make up for liquid loss due to drag-out. DI water (25 ml) was added three times before plating of the 46th, 115th, and 150th coupons. A white precipitate formed in the bath starting with the first addition of water and increasing with each subsequent addition.
• the spent plating solution was filtered to collect the white precipitate.
• the precipitate was washed with water and air dried.
• a sample of the precipitate was analyzed using conventional inductively coupled plasma analysis which indicated the precipitate to be copper-thiourea complex in which there are about 3 to 4 thiourea molecules complexed to one copper ion.
• An immersion displacement tin plating bath was prepared using 282 ml of "A” solution and 282 ml of "B” solution as defined in Example 1. The bath was used to plate 3 x 3 inches coupons of copper laminate at 25° C. with a 90 second immersion time. The coupons were immersed in a 10% sulfuric acid predip solution prior to immersion in the tin bath. After removal from the tin plating bath, the coupons were rinsed with DI water, dried, and then inspected for the quality of the plated tin.
• a solid replenisher consisting of 11.7 g of tin(II) sulfate and 7.7 g of thiourea was prepared. After the processing of every 50 coupons, 2.73 g of the replenisher was added to the tin plating bath with mixing and heating until it had dissolved. The processing of the next set of 50 coupons was then started. A total of 200 coupons were coated in this manner with shiny, uniform tin. After the coating of 200 coupons with the replenishment, the bath volume was 445 ml.
• the 250 ml portion of the partially spent tin plating bath above was used to coat a total of 400 2 ⁇ 2 inches coupons of copper laminate with shiny, uniform tin. After the coating of each group of 50 coupons, 1.21 g of the solid replenisher was added to the bath with mixing and heating as necessary until the solids had dissolved. At this point, the coating of the next set of 50 coupons was then initiated.

Landscapes

• Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrochemistry (AREA)
• Chemically Coating (AREA)
• Manufacturing Of Printed Wiring (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Electroplating Methods And Accessories (AREA)

Priority Applications (9)

Applications Claiming Priority (1)

Publications (1)

Family

ID=25175108

Family Applications (1)

Country Status (2)

Cited By (15)

Families Citing this family (2)

Citations (6)

• 1991
  • 1991-11-27 US US07/799,134 patent/US5211831A/en not_active Expired - Lifetime
• 1992
  • 1992-11-25 KR KR1019920022346A patent/KR100292944B1/ko not_active IP Right Cessation

Patent Citations (6)

Also Published As

Similar Documents

Legal Events